The present invention relates to a system and a method of controlling a communication system, and in particular, the present invention relates to controlling communication links in a communication system having a limited number of communication channels, wherein each communication link occupies at least one communication channel.
In today""s computerized world, telecommunication networks are increasingly used to transmit data between computers. Telecommunication networks, however, have been designed mainly for the transmission of speech signals and therefore are not well equipped for the transmission of digital data at high data rates. Furthermore, subscribers of telephone networks increasingly demand simultaneous use of both, a computer in a data transmission session and a telephone in a telephone call.
One solution to this problem is to link the subscriber, e.g. his household or his workplace with multiple telephone lines which can be used for data or speech transmission at his discretion.
Integrated Services Digital Network ISDN has been developed to provide a subscriber with multiple lines for independent use. If a user has subscribed to ISDN services, he will have two digital telephone lines with each 56-64 kbit/s for the transmission of speech signals or the transmission of data. The ISDN subscriber could now connect his computer to one of the two so-called B-channels of the ISDN system. Via this first B-channel the ISDN subscriber could, e.g. connect to an Internet access point while he would still have the second B-channel available for making phone calls or being called by an outside subscriber. Further, ISDN provides a so-called D-channel. The D-channel is a general packet data channel. In general, the D-channel may be used for both signaling between a user terminal and the ISDN Local Exchange and user voice/data transmission.
With the demand for ever higher rates of data transmission, in many cases the ISDN subscriber will want to use both B-channels of the ISDN service for a data transmission session between his computer and, for example, an Internet Access Server. Data communication protocols are available, e.g., multi-link PPP, which provide the option to connect and exchange data via multiple individual lines or communication channels.
Consequently, if the ISDN subscriber uses both B-channels of the ISDN service for a data transmission session, there is no line available for receiving an outside call or for making a call towards the outside. A user calling from the outside would receive a busy signal, since no B-channels are available, and the ISDN subscriber connected with his computer will also not have a free channel for making a call to the outside.
Therefore, a problem arises when the ISDN subscriber has connected himself to, e.g., an IAS (Internet Access Server) via both of his only two B-channels of his ISDN service. If the subscriber now suddenly gets a phone call via a telecommunications network, the calling subscriber will experience a busy signal. And most likely the ISDN subscriber wants to avoid this situation. He would like to use both lines for the data transmission session, but he also would like to be reachable by telephone.
In ISDN applications, it is basically possible to use the D-channel for a transmission of user data, for a telephone call or a data transmission session. The basic problem, however, remains the same, i.e., at one point all available channels may be occupied.
To solve this problem, xe2x80x9cCall Waitingxe2x80x9d a so called feature has been proposed in the xe2x80x9cEuropean Telecommunication Standard ETS 300058-1xe2x80x9d, October 1991 from the European Telecommunication Standards Institute. This xe2x80x9cCall Waitingxe2x80x9d service allows the user to send a SETUP request as part of the ISDN protocol via the D-channel even through both B-channels of the ISDN system are occupied. This SETUP request allows a terminal to pick up the call, however, only a terminal using a B-channel at this point in time are allowed to do so. This means that in the above case with a computer connected in a data transmission session via both B-channels, only the computer is allowed to pick up the call.
This is not only inconvenient, since only the computer may be used for a telephone call, it also implies that the computer must be equipped with a microphone and speaker and be compatible with the speech bearer service.
It is obviously desired, that the call may be picked up by any telephone terminal, e.g., by a telephone set next to the computer or in a different room. This, however, is not possible with the above ISDN xe2x80x9cCall Waitingxe2x80x9d service, since a B-channel is not available for the telephone terminals (both B-channels are occupied by the data transmission session).
Further, it is disadvantageous, that the xe2x80x9cCall Waitingxe2x80x9d feature does not help when the ISDN subscriber involved in a data transmission session occupying two B-channels (a 2B-channel session) wants to make an outgoing telephone call. In order to make a telephone call, the user has to disconnect at least one of his ISDN B-channels involved in the data transmission session.
Moreover, the xe2x80x9cCall Waitingxe2x80x9d feature might also disturb the data transmission session, e.g., the multi-link PPP-session, if one of the PPP-connections is suddenly put on HOLD.
In view of the above problems it is an object of the invention to provide a system and method for flexibly assigning communication channels to communication links.
This object of the invention is solved by a method of controlling communication links in a communication system, each communication link occupying at least one communication channel and being used for communications between at least two communication devices, comprising: detecting a request for a first communication link; establishing the requested communication link if free communication channels are available; and if free communication channels are not available, detecting at least one second communication link occupying at least two communication channels; releasing at least one of the communication channels of the at least one second communication link; and establishing the first communication link using the at least one released communication channel.
This object of the invention is further solved by a method of controlling communication links in a communication system, each communication link occupying at least one communication channel and being used for communications between at least two communication devices, comprising: detecting a request for a first communication link; establishing the requested communication link if free communication channels are available; and if free communication channels are not available, detecting at least one second communication link occupying at least two communication channels; sending a disconnect command from a control means to the corresponding communication devices; releasing at least one of the communication channels of the at least one second communication link; receiving a release message from the communication devices; sending a release complete message from the control means to the corresponding communication devices; and establishing the first communication link using the at least one released communication channel.
Further, the object of the invention is solved by a communication system, comprising: a plurality of communication devices; a plurality of communication links for communications between the plurality of communication devices, each communication link occupying at least one communication channel; control means including: first detection means for receiving a request for a first communication link and for receiving a termination message in case a communication link is terminated; second detection means for determining at least one second communication link occupying at least two communication channels, in case a free communication channel is not available; and switching means for controlling the release of at least one communication channel of the at least one second communication link, and establishing the first communication link via the released communication channel.
The invention allows a user to establish a communication link, even if a free communication channel is not available, by detecting at least one second communication link occupying at least two communication channels, releasing at least one of the communication channels of the at least one second communication link and establishing the requested first communication link using the at least one released communication channel.
Subsequently, in case a communication channel becomes available thereafter, the second communication link may be restored to its original state by resuming communication via this or these communication channels.
Advantageously, in case the first communication link is terminated, its at least one communication channel may be released and the at least one second communication link may resume communication via the released channels of the first communication link.
Also, in case the first communication link is terminated, the devices communicating via the at least one second communication link may be notified by control means and instructed to resume use of the at least one released communication channel of the first communication link.
Further, it is possible that at least one of the devices communicating via the at least one second communication link performs regular check-ups of the state of the communication channels and resumes use of a released channel, in order to restore the corresponding communication link to its original state, that is, to the state before at least one of its communication channels has been released.
Advantageously, in order to avoid an erroneous release of communication channels of a communication link using at least two communication channels, a service record may be checked and it may be determined whether the target communication link is marked available for releasing at least one of its communication channels.
The invention may advantageously be also applied to systems employing ISDN (Integrated Services Digital Network), e.g., in case data processing units are involved in a data transmission session and conduct a multi-link PPP-session via the two ISDN-B-channels, and an outgoing or an incoming telephone call occurs.
The invention may also be applied to mobile communication networks, in which case the communication devices include mobile communication units of the mobile communication network.
Further advantageous embodiments of the invention are disclosed in further dependent claims.